Why the forecast was so far off for Portland's epic snowstorm

There's no question that Portland woke up on the receiving end of a snowy sucker punch Wednesday. But if you were to look at the forecast on Tuesday, which called for between 1 and 4 inches of snow, you'd have no way to know that more than a foot of powder would blanket the northern Willamette Valley less than 24 hours later.

So what happened? How did the forecast jump from 1 to 4 inches to 8 inches to more than a foot in a matter of hours?

"It was a major bust," said Cliff Mass, a professor of atmospheric science at the University of Washington. "One of the worst failures we've had in a long time."

Yes. We under forecast this event. We saw 6" as the very max possibility. This storm wayyy over-performed. https://t.co/ZyV728NLTk

Forecasters rely on numerical weather prediction models, which use data from weather sensors to predict what's coming. They couple that with experience, said Bill Schneider, a science and operations officer with the weather service for the last 22 years, and develop what they believe to be the most likely scenario.

In the case of Tuesday's overnight storm, Schneider said they saw it coming nearly a week ahead of time. The only part they missed, he said, was the intensity with which the snow would fall and exactly how much of it would pile up.

"We knew as early as last week that there was the potential for this type of storm," he said.

Schneider said that there is no such thing as an easy snowstorm to predict, and a couple of factors came into play to produce the historic blanketing witnessed in the Willamette Valley.

The valley itself is broad and bordered on either side by mountains, the coast range to the west and the Cascades to the east. Complicating things is the Columbia River Gorge, a narrow geographical trough that funnels cold air from the east directly into the northern end of the valley. This complicated terrain makes for unpredictable weather and a high degree of variability, Schneider said.

On Tuesday afternoon, an area of low pressure moved onshore along the central Oregon Coast and began making its way up the valley. Low pressure systems draw in air from around them and the storm began sucking in air from the gorge, but this system was high enough in the atmosphere that it also drew in frigid air from over the Cascades, Schneider said, a rare occurrence.

So you've got cold air coming in from the north and east colliding with warm air filled with moisture from the south. The boundary between the two, called a front in meteorological terms, is where the action happens.

"That pushing together causes upward motion," Schneider said. "In this case it was really strong, as strong as I've ever seen it. We were seeing accumulations of up to 2 inches an hour in some places."

The storm was also accompanied by so-called "thunder snow," Schneider said, a weather phenomenon common in the midwest, but rarely seen in the Pacific Northwest. The low pressure system found a spot it liked over the Portland metro area and refused to budge, pounding the region for much of the night.

The result was a foot-deep blanket of snow Wednesday morning, the likes of which Portland hasn't seen for years, despite the mild forecast of 1 to 4 inches 24 hours earlier.

Mass attributed the missed forecast to flaws in the models forecasters rely on.

"We didn't see any of that in the forecast," he said. "We had a low (pressure system) that just parked itself over northwest Oregon. The models just didn't get it right."

While Mass was quick to call the forecast a "failure," he said the fault lay on the shoulders of the weather models, not the forecasters themselves.

"Human beings can't out-forecast the models," he said.

Schneider said there wasn't one thing the weather service could have done differently that could have better predicted the onslaught of snow and noted that forecasting has greatly improved in the two decades he's been doing it.

"We always know it will be difficult," he said. "We want it to be a perfect forecast, but it's just not possible.

"I was joking with another forecaster the other day who said 'This is going to be a tricky one to forecast,'" Schneider recalled. "I said 'They are all tricky.'"